1. Field of the Invention
The present invention relates to comminution apparatuses and, in particular, to an apparatus for comminuting particles of material in a batch processing operation.
2. Description of the Invention Background
It has been recognized that in various applications throughout modern society, it is necessary to convert particles of materials from a larger size to a relatively smaller size. For purposes of illustration throughout the instant specification, exemplary references will be made to particles of a mineral such as coal which must be reduced to a fine size. It is known that if coal particles are reduced to a mean size range of 5-40 microns, state of the art benefication apparatuses may be employed to effectively separate undesired minerals from the coal particles. The coal particles emerging from the benefication apparatus would be of such a high quality and low sulfur content that they could be used to provide a source of fuel for power generating plants.
Heretofore, no apparatus was available that was capable of efficiently and economically reducing the mean size range of coal particles of, for example, 4 mesh, to the range of 5-40 microns while eliminating virtually all oversize particles. If a process was available to economically effect such a decrease in coal particle size, much of the coal fines that are presently being discarded could be processed to provide a source of fuel for electric power plants. Also, a significant portion of the national reserves of coal which were previously unusable would be suitable for usage. The present invention provides a batch processing apparatus for reducing the size of coal particles to the desired size range by means of an improved vertical stirred wet ball mill.
The prior art contains various examples of vertical stirred wet ball mills for comminuting materials. For example, in Szegvari (U.S. Pat. No. 3,131,875), a vertical cylindrical vessel is provided having a closed bottom and an open or closed upper surface. A vertical shaft is provided concentrically within the vessel and has affixed thereto a symmetrical plurality of radially extending rods or discs. A grinding medium such as steel balls is provided within the container to a level above the uppermost rods.
In the operation of such an apparatus, the shaft is rotated thereby causing the rods to move in a circular path. A slurry comprising the material to be comminuted and water is injected into the vessel so that the slurry fills the volume of the vessel beneath and above the level of the rods. As the rods are rotated, the material particles are trapped between the rods and the balls thereby resulting in the breaking of the particles between such relatively harder materials. In addition, the movement of the rods imparts movement to the balls thereby causing them to collide with one another. As a result, any material particles disposed between the balls will be impacted by the balls and broken. Further, any particles trapped between the balls and the wall of the vessel will be similarly impacted and broken. Prior art stirred wet ball mills typically operate with agitator tip speeds in the range of 1 to 3 meters per second. Due to the use of such relatively slow speeds, the particles are comminuted by attrition, that is, the breakage of corners from the particles rather than a shattering of the particles.
Based on his study of prior art vertical stirred wet ball mills, Applicant has discovered a significant flaw in the prior art mills which has prevented them from achieving the consistently narrow range of extremely small particle sizes required to justify commercial operation of such an apparatus. Applicant's study of the dilatency of the comminuting media and the rheology of the circulating slurry provides an explanation for the problems associated with prior vertical wet stirred ball mills. As the agitating rods rotate through the comminuting media mass, the rods cause the media mass to be disrupted thereby causing its volumetric expansion within the comminution vessel. Because the media mass is constrained in all directions except upward, it must expand in that direction. As such, the media mass tends to expand upward along the vessel side wall. Due to the centrifugal forces imparted on the media mass by the rotating rods, the centrifugal moment on the media mass is toward the vessel side wall. This centrifugal moment causes the media balls to be in closest proximity to one another along the vessel side wall. The centrifugal moment thereby forces the media balls along the vessel side wall to be forced upward thereby causing a conical vortex to be formed within the center of the media mass. The effect of gravity on the media balls located at the uppermost portion of the vortex, that is, against the side wall, causes them to roll down the inner surface of the vortex in an effort to rejoin the comminuting media.
However, in order for the particles contained in the slurry to be effectively comminuted, all components of the slurry must be introduced into the vortex in the center of the comminuting media. The slurry must be introduced into the center of the comminuting media to allow all particles to pass radially outward toward the vessel wall so that such will encounter the maximum amount of comminuting media during the slurry's circulation. In addition, due to the above-described phenomenon of the comminuting media being most dense adjacent the vessel wall, the circulating media is least dense at the center of the vortex. Accordingly, less effort is required to introduce the slurry into the comminuting media in the vortex thereof.
Applicant has also noted that as comminution takes place, the viscosity of the slurry increases. This is so because there are more smaller particles of material in a given volumetric unit of slurry liquid. As those elements of slurry which have passed radially outward from the agitator shaft through the comminuting media to the outer wall of the vessel have been subjected to a complete comminution pass, the elements of slurry adjacent the chamber wall will have a relatively higher viscosity. Due to the continuous centrifugal force toward the vessel wall by virtue of the rotating agitator rods, the more viscous portion of the slurry adjacent the chamber wall will be also forced in the only unrestrained direction, namely upward. When a more viscous slurry component reaches the uppermost portion of the vortex, gravity causes the viscous slurry to be drawn downward along the upper surface of the vortex. However, Applicant has recognized that the flow of viscous slurry components toward the center of the vortex tends to prevent the less viscous and less comminuted slurry components from entering the vortex for subsequent comminution. Therefore, Applicant has discovered that the prior art vertical wet stirred ball mills tend to repeatedly recirculate the more comminuted, more viscous slurry components through the comminution process rather than introducing the less comminuted, less viscous slurry components into the vortex for comminution. Accordingly, complete comminution of the slurry cannot effectively occur. An additional problem has been discovered in connection with prior vertical wet stirred ball mills. Due to the ineffectiveness of such apparatus, they must be operated for significant lengths of time thereby generating excessive heat. Such heat must be removed by complicated and costly liquid cooling means.
In an effort to increase the throughput of comminuting apparatuses, those skilled in the art have turned to an alternative form of comminution apparatus. In such an apparatus, a horizontal comminution vessel is employed to allow the continuous processing of a slurry containing the material particles to be comminuted. A horizontal agitator shaft is supported by the closed opposing ends of the comminution apparatus and a motor drive means imparts rotary movement to the shaft. Comminution rods or discs are attached to the agitator shaft and the comminution vessel is filled with a comminution media such as steel balls.
In the operation of such a horizontal continuous stirred ball mill, the shaft is rotated at a high speed, e.g. agitator tip speeds of 7-10 meters per second, and the slurry is pumped into one end of the vessel. The comminution of the material particles takes place as the slurry travels the length of the vessel. By exposing the material particles to the violent action of the agitators and media along the length of the vessel, comminution takes place by means of shattering the particles.
It will be readily appreciated, however, that numerous difficulties are present in horizontal continuous comminuting apparatuses. Due to the inclusion of a horizontal agitator shaft, intricate seals must be provided at the ends of the vessel which have increased capital costs and have provided significant maintenance problems. Also, due to the rapid tip speeds required, the excessive heat generated must be dissipated by complicated liquid cooling means, thereby adding initial capital and maintenance expenses.
Also, it has been found that due to the high operating speeds of continuous horizontal wet ball mills, the comminution takes place due to the impact upon the particles thereby causing their shattering. However, the violent comminution occurring in continuous horizontal ball mills causes excessive wear rates of the apparatus components and media thereby requiring their frequent replacement. Also, due to the fact that any given particle is only afforded one pass through the apparatus for comminution, if a particle is not comminuted, it will exit the apparatus along with the correctly sized particles and cause problems to subsequent processes.
Applicant's invention relates to a means for improving particle circulation along the agitator shaft in order to introduce the particles into the media vortex. While circulation means along an agitator shaft have previously been provided on vertical machinery in which no comminuting media is present, such apparatus is directed to an entirely different application than Applicant's apparatus. Due to the lack of the comminuting media in such apparatus, the problems discussed above relating to the introduction of particles into the comminuting media vortex are simply not present.
The subject invention is directed toward an improved means for comminuting materials which overcomes, among others, the above-discussed shortcomings in prior art vertical stirred wet ball mills and which is effective to economically reduce the mean particle size to the required level and provide a narrow range of particle sizes. Due to Applicant's inclusion of an additional circulation means, of a form which has not been previously employed in prior art wet ball mills, the instant invention provides solutions to the problems present in prior art vertical stirred wet ball mills and provides a commercially viable apparatus.